A photocoupler includes a light emitting diode (LED) and a light receiving element which are contained in one integrated package. The photocoupler transmits light signals from the LED to the light receiving element. The LED and the light receiving element are typically arranged so as to face each at fixed distance. An “inner mold resin” is disposed in the light path between the LED and the light receiving element and is also used to at least partially encapsulate or seal the LED and light receiving element.
An LED for a photocoupler typically emits an infrared light. The LED has a light emitting layer that is disposed on a gallium arsenide (GaAs) substrate or the like. If the light emitting layer comes into direct contact with the inner mold resin, the light emitting layer will receive stress (due to differences in thermal expansion coefficients or the like) that might cause the light emitting layer to crack after thermal cycling events. Thus, the light intensity output by the LED and/or the lifetime of the LED may be reduced.
To address this concern, some portion of the light emitting layer is often first covered with an encapsulation resin material different from the inner mold resin, and then the inner mold resin is disposed on the encapsulation resin. The encapsulation resin material is a thermosetting or thermocurable resin such as a silicone resin that can be formed by potting of the uncured resin.
In general, multiple LED components are simultaneously formed on a single wafer then diced into separate LED chips. In constructing a photocoupler, an LED chip is electrically connected to a lead frame or a terminal element using bonding wire or the like. After the bonding wire is bonded to the LED chip, the encapsulation resin is disposed on the LED chip. The forming of the encapsulation resin on the LED chip at this stage introduces additional process steps and complexity. Therefore, in some instances, the encapsulation resin may not be formed correctly on the LED chip but rather might be formed on the light receiving diode, or the encapsulation resin may be omitted by mistake.